jQuery.widget: Document that widget event names are lowercased

[tjvantoll]

Fixes gh-182

[gnarf]

Should we use an even example that is camelCased, do we have any?

[tjvantoll]

There are only a couple - e.g. sortable's beforeStop event.

Since progressbar is the widget used to document the widget factory, and it has no camel cased events, I worry that an example would interrupt the flow of this too much.

[tjvantoll]

I'm going to land this as is for now. If someone can come up with a reasonable way of adding an example into the docs without messing with the flow then we can add that later.